Recent evidence suggests that there is a biological disposition in humans for a heterosexual or a homosexual orientation. This disposition may result from the influence of sex hormones during the period of prenatal development. Twin and family data support the idea that sexual orientation is familial but no adequate family study has tested this idea. A study is proposed to ascertain sexual orientation, cognitive style, "masculinity-feminity", presence of affective disorder and alcoholism in a group of homosexual and heterosexual volunteers and their first degree relatives. If sexual orientation does prove to be familial, further studies would be appropriate to test for a genetic basis, e.g., adoptive and linkage studies.